1. Field of Invention
The invention relates to a switching circuit provided with a plurality of bus bars serving as a conductor for coupling a switching element to a power source or a load.
2. Description of Related Art
Generally a metallic bus bar is well known as a conductor having a cross section area large enough to allow electric current to flow between circuit elements or between a circuit element and a power source or between a circuit element and a load in a power circuit such as a switching circuit for an inverter motor. FIG. 8 is a circuit diagram showing a generally employed switching circuit 40 serving as an inverter. FIG. 9 shows a plan view of the switching circuit 40.
Referring to FIG. 8, the switching circuit 40 takes the form of a three-phase inverter having switching elements S1 to S6, such as IGBT (Insulated Gate Bipolar Transistor), and diodes D1 to D6 connected in parallel with the corresponding switching elements S1 to S6. Each of the switching elements S1 to S6 is opened and closed in accordance with a control signal sent from a control unit (not shown). In response to opening or closing of the switching elements S1 to S6, the direct current applied from a direct-current power source 42 is converted into a three-phase alternating current. The three-phase alternating current, then, is supplied to a three-phase inverter motor 44 serving as the load.
Referring to FIG. 9, the switching circuit 40 has power source side bus bars 46 (46P, 46N) between the switching elements S1 to S6 and the power source (not shown), and has load side bus bars 48 (48U, 48V, 48W) between the switching elements S1 to S6 and the respective phases (U-phase, V-phase, W-phase) at the load side. Each of the power source side bus bars 46 is formed into a frame-like shape defined by longitudinal members 46C and lateral members 46R. The longitudinal members 46C are inserted into a substrate 50, which is then subjected to resin molding, so that the longitudinal members 46C are embedded in the substrate 50. Like the longitudinal members 46C, the lateral members 46R are embedded in the substrate 50 except the upper surface. The upper surface of the lateral member 46R is exposed on the substrate 50 as a surface to which wires are bonded for connection with the switching elements S1 to S6. Embedding of the bus bars 46P and 46N in the substrate 50 makes it possible to enhance a flexural rigidity and flexural strength of the switching circuit 40. Furthermore, insulation between adjacent bus bars, for example, bus bars 46P and 46N, or the bus bars 46P, 46N and 48U, 48V, 48W can be secured, respectively.
Each of the load-side bus bars 48 is formed as a strip member that laterally extends on the substrate 50. The switching members S1 to S6 are provided on the substrate 50 between the lateral members 46R of the power source side bus bars 46 and the load side bus bars 48. The respective ends, upper and lower ends shown in FIG. 9, of the switching members S1 to S6 are connected to the lateral members 46R and the load-side bus bars 48 via wires 52.
The wires 52 are bonded to the respective bus bars 46, 48 through ultrasonic bonding. More specifically, in the state where the metallic wires are pressed onto the bus bars 46, 48, each portion of contacts between the metallic wires and the bus bars 46, 48 is oscillated using ultrasound. Accordingly the wires 52 are bonded onto the surface of the bus bars 46, 48.
However, the aforementioned switching circuit formed by insert-molding of the bus bars (power source side) into the substrate has caused various problems. For example, as a flow path of the resin becomes complicated during the insert molding, the resultant switching circuit may be deformed or cracked. Furthermore the inserted bus bars may be deformed by the temperature rise during the resin molding.
Conventionally, adjacent bus bars are kept insulated by inserting the bus bars into an insulating resin, that is, covering the peripheral area of the bus bars with the resin material. However, this may increase the size of the substrate.
When a certain failure occurs during resin molding of the substrate, a gap may be generated in a portion of the substrate on which the lateral member of the inserted bus bar (46R shown in FIG. 9, for example) abuts. This may prevent the bus bars from being insulated, or cause difference in the oscillating state or the temperature rise during ultrasonic bonding between the portion with the gap and the portion with no gap. As a result, the bonding strength or the resistance value may vary depending on the bonded portion.